“The practice or process by which an undesirable
organism is controlled by means of another organism”
In other words, Microbial control
is both a naturally occurring process (which we can exploit)
and the purposeful use of one organism to control another.
“Any agent capable of causing disease in the plants”
The term pathogen is usually restricted to living agents,
and certain insect larval stages.
“The use of micro-organisms to control plant
There are many microbes which are being used for
Three most important which are being used are;
This term was first coined by Harry Smith.
Louis Pasteur also contributed a lot in this field.
• Showed microbes caused fermentation
• Studied spoilage and introduced “Pasteurization” to prevent it
• Used cotton plugs in his cultures to prevent air borne
contamination, devised Aseptic Technique.
It is “Environment Friendly” cause no pollution in the
It tends to keep the natural environment at balance
In practice, Microbial control can be achieved by three
Bio pesticide Approach
Management and manipulation of the environment
Trichoderma are fungi that are present in nearly all soils.
They found in diverse habitats.
In soil, they frequently are the most prevalent culturable
They are classified as Imperfect Fungi.
They found in the form of colonies.
Colonies are often uncoloured But sometimes give Buff,
Yellow, Amber or yellow-Green colour.
It is Bio-fungicide.
Rapid growth in culture.
They reproduce themselves by Chlamydospores hence are a
fastest way to control fungi.
The use of Trichoderma as microbial control was recognized in
Commercial production of Trichoderma for the protection and
production enhancement of crops is in progress including;
United States of America
One of the most characteristics of Trichoderma is their ability to
parasitize other fungi.
It is therefore It is not surprising Weindling ascribed Microbial
control by Trichoderma lignorum of citrus seedling disease,
incited by Rhizoctonia solani, to mycoparasitism.
The mycoparasitism of R. solani hyphae by the hyphae of the
Rizoctonia solani Includes;
Coiling of hyphae around pathogen hyphae
Penetration of hyphae into pathogen
Dissolution of the host cytoplasm
This phenomenon occurred regardless of the supply of
external nutrients to the host or Mycoparasite.
o Although he considered the possibility that under certain
circumstances T. lignorum might act as a competitor for
nutrients with R. solani.
o He much favored Mycoparasitism as the principal mechanism
for microbial control.
Two years later, Weindling reported that a strain of T.
lignorum produced a “Lethal Principle”
Secretion of Antibiotic into the surrounding medium, allowing
parasitic activity by microbial agent is called Lethal principle.
In 1941 the “lethal principle”, demonstrated as it was toxic to
both R. solani and Sclerotinia Americana, and Weindling
named it Gliotoxin.
Later research clarified that it is not T.lignorum, but
Gliocladium virens, a species that has recently been renamed
If mycoparasitism and antibiotics are not the principal
mechanisms in the Microbial control process, Then what?
One mechanism that has gained adherents in recent years is
that of competition through Rhizosphere competence
Rhizosphere competence is important because a Microbial
control agent cannot compete for space and nutrients if it is
unable to grow in the Rhizosphere
Trichoderma species are either added to the soil
Applied as seed treatments
o It grow readily along with the developing root system of the
o This can be shown easily by simply plating surfacesterilized
root segments from treated plants on an agar medium
o After a suitable incubation period, the fungus can be seen
growing from virtually all parts of the root
The difficulty in viewing competition through Rhizosphere
competence is that strains of T. koningii that are excellent
root colonizers exhibit little or no Microbial control activity
against R. solani on cotton seedlings
One concept that is associated with competition and
Rhizosphere competence, the replacement of endogenous
fungi on the root surface, can be difficult to demonstrate.
Trichoderma species are often able to suppress the growth of
endogenous fungi on an agar medium and therefore mask
o If these same cultures are incubated at 40°C, a temperature at
which T. virens will not grow, the pathogen grows readily
from many parts of the root system
o This may also occur with other Trichoderma species and
o But it is not easily demonstrated because growth of the
Microbial control agent can’t be suppressed without
suppressing the pathogen
• One idea that has been advanced is that enzymes such as
chitinases and/or glucanases produced by the Microbial
control agent are responsible for suppression of the plant
• These enzymes function by breaking down the
polysaccharides, chitin, and glucans that are responsible for
the rigidity of fungal cell walls, thereby destroying cell wall
Metcalf and Wilson described the colonization of onion
roots, infected with Sclerotium cepivorum, by T. koningii
Hyphae of the Microbial control agent penetrate into infected
epidermal and cortical tissue of the root to destroy the
hyphae of the pathogen, with little or no damage to
uninfected plant tissue
The protease enzymes break down hydrolytic enzymes into
peptide chains and/or their constituent amino acids and
thereby destroy their capacity to act on plant cells
Lorito further expanded this concept by combining a number
of antifungal compounds with several kinds of hydrolytic
enzymes and applying them to Propagules of B. cinerea and
Synergism occurred in all cases, but the level depended on
the antifungal activity of the enzyme
Synergism was lower when the enzyme was added after the
This Indicates that cell wall degradation was needed in order
to establish the interaction
• Another mechanism proposed to explain biocontrol activity
by Trichoderma species is that of induction of resistance in
the host plant by treatment with the microbial control agent
• Interestingly, the plant defense became muted with time and
began to resemble a symbiotic mycorrhizal association
• Biocontrol activity against R. solani was highly correlated with
induction of terpenoids synthesis in cotton roots by
Trichoderma species, even among strains of T. virens that
were deficient for mycoparasitism and antibiotic production
In addition to terpenoids synthesis, treatment of cotton roots
with T. virens also induced significantly higher levels of
peroxidase activity than that found in control roots
Peroxidase activity and terpenoids levels in seedling
hypocotyls were not significantly different from those found
in the controls. In this case, plant defense responses
appeared to be confined to the root system
One unique mechanism employed by Trichoderma species to
effect biological control that does not fit neatly into any of
the categories previously mentioned was recently discovered
Disease control could be effected by wild-type strains or by
mutant strains that were deficient for mycoparasitism,
antibiotic production, and induction of terpenoids synthesis
in cotton roots
If, however, pathogen Propagules were induced to germinate
by artificial means, none of the above treatments gave
effective control of the disease
Trichoderma species exhibit other characteristics during
interactions with host plants that may contribute to disease
resistance or tolerance
These characteristics manifest themselves by increases in
plant root and shoot growth, resistance to biotic and abiotic
stresses, and changes in the nutritional status of the plant
At maturity, the treated plants had larger stem diameters and
increased yields of grain and silage
o The results have shown that not all the mechanisms and
characteristics deemed necessary for optimum Microbial
control are found in the same organism
o Very often those strains that have the capacity to produce
enzymes and antibiotics that are associated with Microbial
o There are not the ones that have good storage qualities or
function well at temperature and moisture levels where
The mechanisms employed by biocontrol agents to effect
biological control of plant diseases are many and complex,
and their use varies with the kind of biocontrol agent,
pathogen, and host plant involved in the interaction
Mechanisms are also influenced by the soil type, by the
temperature, pH, and moisture of the plant and soil
environment, and by other members of the micoflora
Our knowledge of the complexity of these systems is
currently limited by our ability to perceive them, and a great
deal of research will have to be undertaken in order to fathom
exactly what is taking place during the Microbial control
What we observe and define Microbial control
The combination of a number of different
mechanisms working synergistically to
achieve disease control